The present invention relates to devices for the feeding of sheets of wrapping material to packaging machines, particularly to packaging machines which operate in a non-continuous manner at consecutive cycles and in which the feeding of the sheet is automatically inhibited for every cycle in which the object to be packaged is not present or it is rejected, for example because it has been found defective.
The intermittent motion groups of the sheet feeding devices of the above mentioned type, up to the present date have been mechanically driven by deriving their motion from the packaging machine, for example by means of mechanisms employing Geneva movements or cams, or by means of clutch and brake mechanisms of electromagnetic type, or the like. This type of drive presents the inconvenience that the modification of the length of the sheets requires the modification of a mechanical ratio of a transmission, or of clutch connection and disconnection times. Moreover, whenever the feeding must be inhibited for a cycle of the packaging machine, due to the absence or rejection of an object to be packaged, the mechanical elements for the transmission of the drive from the packaging machine to the intermittent motion group or groups of the sheet feeding device are subjected to sudden and severe stresses at the moment of stopping and at the moment of re-starting the said groups.
When the sheets are obtained by transversal cuttings from a continuous pre-printed web, presently known feeding devices require complex electromagnetic means for maintaining the correct phasing between the printing of the web and the cutting thereof. Of course, the above mentioned inconveniences become greater and more serious, upon increase of the speed of operation of the packaging machine.